Current rigid endoscopy technology makes use of two optical channels, one used for light delivery and the other used for image extraction. A significant flaw to this solution is that the effective area of each channel is roughly half of the cross-sectional area of the endoscope. Where the first lens is a negative lens, this ratio can be worse. In other words, only a small portion of the negative lens is used for image collection, while the rest of the surrounding area is left unused thereby wasting surface area on the tip of the endoscope that could otherwise be used for light delivery. Thus, when separate channels are used for image extraction and target illumination, only half of the scope area is effectively used to collect the image, while the other half delivers light for object illumination.
From a physics standpoint, however, there is no limitation of sending two beams of light through the same optical channel for systems composed of linear elements. In such an arrangement, the two beams do not interfere with each other, and there is no image disruption, even if the image is superimposed over the illumination beam. The difficulty with such an endoscopic solution is that classical illumination systems are realized in the visible part of the spectrum. Thus, when the illumination beam is sent through the imaging lenses, the intensity of the reflected portion of the illumination beam overcomes the image beam by orders of magnitude, preventing the scope from rendering any useful image.